This invention relates to a method of and apparatus for reducing back corona effects in the process of charging particulate material in an electrostatic precipitator. More particularly, the present invention relates to a method of and apparatus for reducing the effects of back corona so that a substantially unipolar ion field is maintained for charging particulate material in a charging stage upstream from a particulate material collecting stage.
Many conventionally applied methods and apparatus for control of back corona are based principally upon reducing the resistivity of particulate matter collected by electrostatic precipitation. Conditioning reagents are sprayed into the gas stream or introduced into the boiler along with the coal. These methods require special systems for handling and dispensing chemical conditioning agents in large quantities.
The effectiveness of the invention does not depend upon the chemical nature of the particulate material to be precipitated. No chemical agents are required.
It is known from U.S. Pat. No. 2,142,129 entitled "Apparatus for Electrical Precipitation" to Hoss et al. to pass gas with entrained particulate material into an apparatus which includes an external housing, a centrally located corona discharge electrode and an apertured intermediate electrode, the intermediate electrode being held at a potential less than but of the same polarity as the discharge electrode. The gas in which is entrained the particulate material passes radially through the apertured electrode towards the internal surface of the housing. The housing acts as the dust collecting electrode and the gas is passed out of the apparatus via an opening between the housing and the apertured electrode. Apparatus of this general type have the disadvantage that dust which collects between the apertured electrode and housing tends to cause electrical discharges and the probability of back corona discharge is also considerable.
It is known from U.S. Pat. No. 1,605,648 entitled "Art of Separating Suspended Matter from Gases" to M. W. Cooke to pass gas with entrained particulate material into an apparatus which includes an external housing, a centrally located corona discharge electrode and an apertured intermediate electrode, the intermediate electrode being held at the same polarity and potential as the housing with respect to the discharge electrode. The gas with the entrained particulate material is passed into the apparatus, the particulate material collecting on the outside of the intermediate electrode in the neutral zone between this electrode and the housing with the gas passing outwardly via a circumferential opening between the discharge and apertured electrodes. Here again, the disadvantages of back corona and electrical discharges exist.
Electrostatic precipitators are known which include a charging section or stage having a passive electrode and a corona discharge electrode between which gas containing particulate material is passed, the passive electrode surrounding the discharge electrode. The particulate material becomes charged, as a result of the corona discharge, and with the gas is passed to a further stage provided with an electrical field which effects precipitation of the charged particulate material. An example of precipitators of this preconditioning type can be seen in U.S. Pat No. 3,747,299 entitled "Electrostatic Precipitator" to Ta-Kuan Chiang and the patent to Hoss et al, supra. It is believed that the possibility of sparking is reduced, as compared with the apparatus disclosed in the patent to Cooke, supra, because dust and other particulate material are not collected on the passive electrode. Nevertheless, the disadvantage of back corona discharge is present and, when taking place, can seriously reduce and even virtually completely counteract the desired charging of the entrained particulate material.
The techniques of charging particulate material in electrostatic precipitators as noted above depends upon the presence of ions, principally of a single polarity, in the charging region. If comparable quantities of both positive and negative ions are present, very poor particle charging results, because of the competing effects of the oppositely charged ions. In a conventional electrostatic precipitator a layer of high resistivity particulate material on the passive electrode may suffer electrical breakdown, resulting in a corona discharge from the surface of the passive electrode. This phenomenon, known as back corona, produces an undesirable bipolar ion field in most of the space between the electrodes of the corona system.